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This topology works when a node is connected to exactly two other nodes in the network forming a continual path for dataflow through each node, forming a ring.
Since the availability of each node is vital to the network, the failure of a connection can bring the whole network down. Because of this, many ring networks send data over a second ring forming a redundant link or a 'dual-ring' network. Such is the technology of FDDI.
The most common type of network, this layout consists of a central device (hub, switch or computer) that is the intermediary for transmitting data to the other nodes.
Unlike the ring, failure of one node does not bring the whole network down; but if the central device fails, then be ready for some downtime.
The simplest type of network, the clients are connected together using a shared line or link called a bus.
Though it is simple, since the line is shared, the bandwidth reduces drastically when additions are made to the network. But the biggest problem is when two clients transmit data signals at the same time. This is when data collisions take place and data loss occurs.
This is the most redundant yet equally complicated of all the four mentioned. It works on the principle of 'everything connected to everything' and taking it to its literal sense. All nodes are connected to each other via a point-to-point link. A broken link or two will not affect the connectivity of this network.
Though it sounds quite intriguing, laying the cables is an expensive and a strenuous task and is not feasible for large networks.
Two types of media can be used for both topologies:
Twisted Pair cable (TP)
This type of cable consists of many pairs of wires with each individual twisted around the other in the pair. This is done to cancel the electromagnetic interference (EMI) from external sources and crosstalk between adjacent pairs.
The most famous type of TP cable is the Unshielded TP (UTP). They are found in many Ethernet networks and telephone lines.
Fibre optic cable (FoC)
This cable contains one or more optical fibres.
Twisted Pair cable (TP)
Thin and flexible cable, can run through walls easily
Defence against EMI greatly depends on the number of twists per foot; less twists, lower defence
Low range, maximum of 100m.
Fibre optic cable (FoC)
Immune to EMI
Very high bandwidth with a range up to 10km.
In conclusion, looking at the types of networks and the media, the best options for the fire department is a star Ethernet network over Unshielded Twisted Pair (UTP) cables.
The fire department will require an internet connection for three main purposes:
As the name says, employees will need to browse the web, for personal or professional reasons. Most operating systems support this service, with web browsers built into them.
This service will be used to host the fire department's internal and/or external websites.
Looking at the above internet requirements, the following assumptions can be made:
All employees will not be browsing and/or reading their emails at the same time
Web publishing will require a dedicated broadband connection, so that the public can contact the fire department, make complaints and report fires through its online service (assuming they have that functionality) without suffering any webpage errors.
Therefore, employees will need a 1 Mbps connection, while the web server will require another connection of the same bandwidth (1 Mbps) for itself.
Two service providers can provide this service, Du and Etisalat, both of which price their services at same levels. Since Etisalat has been operating for a long time, they have a much stronger infrastructure therefore they are a good choice.
Technologies for communication
For communication purposes, the Fire Department can use a 'Two-way Radio'. A two-way radio is a radio that can both transmit or receive radio signals (also known as a transceiver). These are available in mobile, stationary-base and portable hand-held configurations. The hand-held ones are often called walkie-talkies. All these come with a push-to-talk button, which is used to initiate the communication.
Two-way radio systems can be applied in many situations like in aviation or the marines; the following are examples of lad-based radio systems:
Professional Mobile Radio (PMR)
Also known as Private Mobile Radio (PMR) in the UK and Land Mobile Radio (LMR) in North America, this uses portable, base stations and dispatch console radio systems. This type of system is commonly used in police departments. Important features of PMR include:
Point to multi-point communication
Push-to-talk system, where a button initiates the communication; indicates that it is half-duplex
Large coverage areas
Closed user groups
Uses VHF or UHF frequency bands
Each user has a dedicated frequency for communication
Trunked radio systems
Also known as Public Trunked Mobile Radio (PTMR), it was designed to overcome the disadvantages of the Professional Mobile Radio (PMR). Though most of the features are similar between PMR and PTMR, the only difference between them is that PTMR does not require the person to have a dedicated frequency for communication.
Trunked radio systems are used because they can have virtually unlimited users over fewer frequency bands. Some PTMR radio equipment have data ports, allowing data to be transmitted over the network.
Global System for Mobile Communications (GSM)
This is the most popular standard for mobile communications. It works by means of searching for cells around the area. There are five different cells sizes in a GSM network. The coverage area of each cell depends on the implementation and environmental factors. The radius of the cell depends on the antenna height, propagation and antenna gain.
GSM operates in the following frequency bands:
Each GSM mobile device requires a Subscriber Identity Module, most commonly known as a SIM card. This card is detachable and contains the user's subscription information and a phone book.
GSM provides two-way communication (full-duplex)
Short Messaging Service (SMS) is also provided; this allows sending short text-based messages to handsets
Data-transfer speeds are clocked at 9.6Kbits/second.
As a response to this mediocre data rates, general Packet Radio Service (GPRS) was introduced to take advantage of the current GSM network and provide a much higher data rate ranging from 14Kbits/second to 112Kbits/second. This in turn allows for reasonable Internet access because GPRS uses an IP backbone.
GPRS also supports Quality of Service (QoS) and this ensures adequate bandwidth availability.
The Telecomm Regulatory Authority (TRA) in this country provides an extensive and comprehensive document on the legal requirements regarding PMR and GSM networks.
For frequency spectrum authorization, the applicant should do the following  :
Fill an application form (soft or hard copy)
Any technical details that might be required by the TRA
Copy of a valid commercial license or official letter of registration
Proof of payment for processing the application (an amount that equals 500 Dirhams)
Applicant should use the equipment that meets the TRA's standards
Applicant will apply for the minimum spectrum resource required
This authority to use the frequency is normally valid for one year and can be valid for a period of up to five years, which is when the holder of the frequency will be asked to renew his authorization. 
The annual spectrum fees for Public Land Mobile (cellular services including GSM, UTMS and IMT) is  :
[FF * CF * P * BW] / 4000
FF is the Frequency Factor
CF is the Coverage Factor
P is the price per MHz (currently at 978,560 Dirhams)
BW is the bandwidth assigned in MHz
The annual spectrum fees for PMR, Paging, PTMR in the frequency range of 30MHz - 700MHZ is  :
NC * CF + SUM(WE * 500 * PF)
NC is the number of channels that will be assigned
WE is the number of wireless equipment that will be included
PF is the power factor of the device
CF is the coverage factor
SUM(WE * 500 * PF) is the sum of all wireless equipment multiplied by 500 multiplied by the power factor of each device
Installation and licensing cost
The costs that will incur during setting up and running the equipment are:
Purchasing the equipment
Obtaining licences for the frequencies that will be used
The licencing costs will depend on a number of factors such as the number of frequency bands that will be used, the number of channels that will be assigned, range, etc.
Maintaining the equipment
This will include costs of operation, the people employed to take care and run the equipment, repairs that might need to take place, electricity and so on.
Limitations of the methods described previously:
Radio systems must be in effective range over each other
Channel bandwidth limits number of simultaneous conversations
Communication is not encrypted for PMR but it is in PTMR hence eavesdropping PMR networks is easy
Only one-way communication is possible
Other radio devices might interfere with the signals, example mobile phones
Data transfer rates are slow
Expensive to create a new GSM network
Signals are not encrypted
Encryption mechanisms for GSM networks are very expensive
If an existing carrier network is used, data transfer rates are high but so is the cost
Though GSM networks provide full duplex communication with faster data rates, PMR is still a better choice because it is relatively inexpensive to setup and operate and provides just enough functionality for the Fire Department to keep in touch with their mobile units.
Three vendors; Panasonic, NEC and Siemens were selected as providers for a PABX system. The features and cost are as follows:
NEC Univerge SV8100
Siemens HiPath 3350
156 max extensions
20 single line telephones
128 max IP phones
64 max portable handsets
96 BRI and 46 PRI channels
104 physical ports
16-bridge multimedia conference
8 portable handsets
96 max IP phones
36 single line phones
16 portable handsets
The NEC Univerge is a good option because it supports Voicemail, ISDN, multimedia conferencing (audio and video) and more than enough ports for the employees. It is also modular, which means adding additional units for phone lines is easy when it is time for an upgrade.
For a sector such as the Fire Department, it is crucial that it stays alive no matter what the situation, whether it be a power outage, a natural disaster, communications going down etc. and there should always be a backup plan in the case that any of these happen. The list below is by no means exhaustive, but does contain a few measures that can be put in place:
UPS, Generators, Solar Panels
Each server, radio base station and telephone exchange should be connected to an Uninterrupted Power Supply, or a generator or a battery connected to a solar panel. There are no restrictions on how many of these can be used. This will ensure that communication is not hindered.
All data centres should have specifications against fires and terrorist attacks.
If data must be transferred, the Fire Department can use satellite or GSM networks. They might not have their own GSM network, so they can use the carrier network to ensure on-going communication.
Fax machines should be preprogramed to divert calls.
As mentioned above, all data centres must have specifications against fires or terrorist attacks. Natural disasters cannot be avoided but the risk can be reduced. As such, if there comes a point that something should happen, all data held within the servers must be backed up and stored in a different location altogether. This will mitigate data loss and recovery will be relatively easier.